It is often necessary to connect optical fibers end-to-end and various techniques have been developed for doing so while maintaining precise coaxial alignment of the fiber cores so as to keep connection (insertion) losses low. When the connection is to be made “in the field”, for example between two previously-installed “field” fibers, a mechanical splice often is preferred, since it is generally much easier to perform a mechanical splice than a “fusion splice” in the field. Known such mechanical splices normally abut opposed ends of the optical fibers coaxially within the device, usually with index-matching material, e.g., a gel, between the opposed ends so as to improve matching. The fibers are retained in position by locking means at opposite ends of the body of the mechanical splice.
Where a fiber is to be “connectorized” by adding a connector to one end, a mechanical splice-based connector may be used which, in effect, constitutes one half of a mechanical splice device. Generally, such a mechanical splice-based connector comprises a ferrule through which one end portion of a fiber stub extends. The other end of the fiber stub is mechanically-spliced to the end of the fiber to be “connectorized”. The term “mechanical splice-based connecting device” used herein embraces both mechanical splices and mechanical splice-based connectors.
When such a mechanical splice-based connecting device has been installed, it is desirable to be able to verify the quality of the splice, specifically by estimating the insertion loss attributable to the splice. Typically, the insertion loss must be measured to within 0.2 dB of actual measured insertion loss, preferably to within 0.1 dB average error, with a certainty of 95 percent. It is also desirable to be able to measure such insertion loss in a simple manner that can be used to check the integrity of field-installed mechanical splice-based connecting devices.
In one known type of field-installable mechanical splice-based connecting device, the connector body around the splice is translucent so that light leaking from the splice can emerge from the connecting device. It is known, when using such a connecting device that comprises a mechanical splice-based connector, to check the integrity of the splice after the fibers are “locked” into place by launching visible light into the splice by means of a jumper fiber attached to the ferrule of the connector, and observing the glow around the translucent body portion caused by light leaking from the splice. If the level of the glow is deemed to be excessive, the user may unlock and withdraw the fiber, and then repeat the steps to attempt to reduce the glow. If the glow is still excessive, the user will normally change the mechanical splice-based connector, re-cleave the fiber, and repeat the above steps. For more information about such connectors and the method of installing them, the reader is directed to U.S. Pat. Nos. 6,816,661 and 6,931,193, both of which name Barnes et al. as inventors. The contents of these patents are incorporated herein by reference.
A limitation of this “glow-observing” approach is that the determination of what is “dark” is very subjective, and its efficacy varies according to user experience and judgement and to ambient lighting conditions. Also, when connecting two field fibers by means of a mechanical splice, it may not be possible to inject light into the splice from a local light source, so it may be desirable to estimate the insertion loss using the “in-line” light signal propagating in the fiber(s). This light is not visible, however, and it is not straightforward to replace the human operator with an optical system which would capture and quantify the leaked light accurately, yet be simple and robust enough to use in the field.
Hereinafter, reference will be made to splices between first and second fibers, it being understood that the first and second fibers may be regular fibers, for example two field fibers, being spliced together by a mechanical splice, or a regular (e.g., field) fiber and a fiber stub of a mechanical-splice-based connector.